Hi. Note that if you drill down, the documentation for key reads:
This property is no longer available and will be removed from the documentation soon. See the deprecation notice (https://developer.atlassian.com/cloud/jira/platform/deprecation-notice-user-privacy-api-migration-guide/) for details.
See also: Announcement: Reminder on removal of user names
And this post from October 2018: https://developer.atlassian.com/cloud/jira/platform/deprecation-notice-user-privacy-api-migration-guide/